Method of manufacturing a lamination stack for use in an electrical machine

ABSTRACT

A lamination stack for use in a rotating electrical machine includes a plurality of sheets of ferritic material. Each of the sheets has first and second sides that include asperities, and the asperities have a height of about two microns and a width of about two microns. A layer of electrically insulating material is provided between adjacent pairs of the ferritic sheets in the stack, and the asperities extend into the electrically insulating material.

CROSS REFERENCE

This application is a division of U.S. application Ser. No. 15/597,275,filed May 17, 2017, which claims priority to German patent applicationno. 10 2016 208 744.4 filed on May 20, 2016, and the entire contents ofboth applications are fully incorporated herein by reference.

TECHNOLOGICAL FIELD

The present disclosure is directed to the field of rotating electricalmachines, such as magnetic bearings, which comprise a magnetic bearingstator, a magnetic bearing rotor and position sensors. Morespecifically, the disclosure is directed to a lamination stack for usein a rotating electrical machine that operates in a corrosiveenvironment or is required to be NACE-compliant.

BACKGROUND

The use of magnetic bearings in machines such as turbo-expanders andcompressors is becoming increasingly common, due to the advantagesassociated with being able to operate the bearing in the process gas ofthe machine, without the need for sealing. The bearing stator generallyincludes stator laminations incorporated within stator poles and copperwings. When energized, the bearing stator tends to attract the bearingrotor, on the basis of the Lenz-Faraday principle. The bearing rotoralso comprises laminations made of soft magnetic material. Thelaminations are often referred to as lamination stacks and areadvantageously made of soft magnetic material with excellent magneticproperties. Silicon-iron is a material that is commonly used inlamination stacks. In a corrosive environment, however, such a materialcannot be used without protective measures.

One commonly applied protective measure is to encapsulate the bearingstator and the bearing rotor, so as to isolate them from the processgas.

To protect the rotor laminations and rotor shaft, a solution is proposedin CA 2,624,347 in which selected exposed surfaces of the rotor shaftare provided with a barrier layer. The application of the barrier layerincreases the complexity and thus the cost of the manufacturing process.

In U.S. Pat. No. 9,000,642, a solution is proposed in which the statoris encapsulated by a corrosion resistant jacket and the rotorlaminations are made of a magnetic anti-corrosion material such asferritic stainless steel. The encapsulation of the stator enables theuse of silicon-iron stator laminations, but the airgap has to beenlarged in order to insert the jacket, which decreases the magneticproperties such as field sensitivity of such devices (actuator, sensor).Also, while the use of ferritic stainless for the rotor laminationsenables the rotor to function unshielded in the process gas, there is acompromise with regard to the magnetic properties.

FR 2888390 disclosed a method of making a laminated magnetic circuitcomprising a stack of sheets of a ferromagnetic material onto which anadhesive is applied by silkscreen, then stacking the sheets, and thencompressing the stack thus obtained to harden the adhesive. Such alaminated magnetic circuit cannot be used in a corrosive environment.

Consequently, there is room for improvement.

SUMMARY

The present disclosure is directed to a lamination stack which hasexcellent corrosion resistance and excellent magnetic properties, sothat the lamination stack may be used unshielded in a rotatingelectrical machine that operates in a corrosive environment.

A first aspect of the disclosure comprises a lamination stack for use ina rotating electrical machine that is formed by a method that includesthe steps discussed below. The steps include: providing a plurality ofsheets of ferritic material that have a first side and a second side anda thickness from the first side to the second side. The first and secondsides have asperities with a height of about two microns. The methodalso includes coating at least one side of each sheet with a chemicallyprotective electrically insulating material, stacking the coated sheets,compressing the stack of the coated sheets, heating the compressed stackat a temperature above a melting temperature of the insulating material,and cooling the compressed stack.

Another aspect of the disclosure comprises a lamination stack for use ina rotating electrical machine that includes a plurality of sheets offerritic material, each of the sheets having a first side, a second sideand a sheet thickness from the first side to the second side. The firstand second sides includes asperities, and each asperity has a height ofabout two microns and, optionally, a width of about two microns. Thelamination stack also includes a layer of electrically insulatingmaterial between adjacent pairs of the ferritic sheets in the stack, andthe asperities extend into the electrically insulating material.

A related method according to the disclosure comprises steps of:

providing naked sheets made of ferritic material;

preparing both sides of each sheet so as to obtain a determined surfaceroughness;

coating one side of each sheet with a chemically protective,electrically insulating material;

stacking the coated sheets;

compressing the stack thus obtained;

heating the compressed stack at a temperature above the meltingtemperature of the insulating material; and

cooling down the compressed stack so as to form an integral laminationstack consisting of a succession of alternating sheets of ferriticmaterial and layers insulating material.

In a preferred embodiment of the invention, the ferritic material is aferritic stainless steel having an alloy composition in accordance withAISI 444 or EN 1.452.

In a further preferred embodiment, the coating of each sheet is done asfollows: firstly the chemically protective electrically insulatingmaterial is disposed on the sheet in the form of solid powder, then thesheet is heated up at a temperature above the melting point of thechemically protective electrically insulating material, and finally thesheet is cooled down in ambient air.

In a still further preferred embodiment of the invention, thepreparation of both sides of each sheet is performed by a chemicalattack, such as by immersion into a hydrofluoric acid bath.

In a still further preferred embodiment of the invention, the chemicallyprotective, electrically material is a fluoropolymer, preferablyFluorinated Ethylene Propylene, because this material doesn't reactchemically with processed gas.

In a still further preferred embodiment of the invention, each sheet offerritic material has a thickness of between 0.05 and 3.0 mm, and eachlayer of chemically protective electrically insulating material has athickness between 5 and 100 microns.

In a still further preferred embodiment of the invention, thecompression force applied during the compression of the stack is between80 and 200 MPa.

The present invention further defines an electrical machine that isequipped with a lamination stack produced according to the method of theinvention. In one example, the electrical machine comprises a magneticbearing and the lamination stack forms part of the rotor and/or statorassembly of the magnetic bearing.

As a result of the excellent corrosion resistance, the electricalmachine may operate in a corrosive environment with a high degree ofefficiency. Other advantages of the invention will become apparent fromthe following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an example of an electrical machinethat comprises lamination stacks manufactured using the method of thedisclosure.

FIG. 2 is a flowchart of the method of the disclosure.

FIG. 3 schematically illustrates a step of compressing the stack with adedicated compression tool according to an embodiment of the disclosure.

FIG. 4 is a magnified view of a detail of the lamination stack accordingto the disclosure.

DETAILED DESCRIPTION

An example of part of a magnetic bearing assembly is shown in FIG. 1,the assembly 10 comprising a housing 20 in which a rotor 30 isrotationally supported by a magnetic bearing. The magnetic bearingcomprises a stator 40 mounted to the housing 20 and disposed about therotor 30. The stator 40 includes a lamination stack 41 andelectromagnetic windings 42, which are wound so as to create a magneticfield when supplied with electric current. The rotor 30 comprises arotor shaft 32 and a rotor lamination stack 31 which is mounted on therotor shaft 32 and aligned with and disposed in magnetic communicationwith the stator 40. When appropriately energized, the stator 40 iseffective to maintain the rotor lamination stack 31 in an unstableequilibrium, so as to provide levitation and radial placement of therotor shaft 32.

The lamination stacks of the stator 41 and the rotor 31 are made offerritic or ferromagnetic material.

Such ferromagnetic material can for instance be Si-Iron, Co-Iron orNi-Iron.

Let us assume that the depicted assembly 10 is part of a system thatoperates in a corrosive environment, such as turbo expander-compressorsystem applied in the oil and gas extraction sector. In order to improvethe corrosion resistance of the rotor laminated stack 31 and of thestator lamination stack 41, it is advantageous to use a ferriticstainless steel. In a preferred embodiment the ferritic stainless steelhas a composition according to AISI 444 or EN 1.4521.

This ferritic stainless steel has an alloy composition that includes:

≤0.025 wt. % carbon (C)

17.0-20.0 wt. % chromium (Cr)

1.75-2.50 wt. % molybdenum (Mo)

from 0.5 to 1.0 wt. % silicon (Si)

from 0.3 to 1.0 wt. % manganese (Mn)

from 0 to 0.8 wt. % niobium (Nb)

from 0 to 0.8 wt. % titanium (Ti)

from 0 to 0.04 wt. % phosphorous (P)

from 0 to 0.03 wt. % sulfur (S)

from 0 to 0.035 wt. % nitrogen (N)

with the balance being iron (Fe), including unavoidable impurities.

For efficient functioning of the magnetic bearing, it is also importantthat the lamination stacks 41, 31 have good magnetic properties. Highrelative permeability and high saturation magnetization are particularlyimportant properties. These properties are optimized thanks to a specialheat treatment, which maximizes the magnetic properties of the ferriticstainless steel without compromising the corrosion resistance, themechanical properties or electrical resistivity.

The heat treatment consists of annealing at a temperature in excess of900° C. followed by rapid quenching. This produces a microstructure thathas fewer precipitates than the untreated base material. Furthermore,the annealing step induces grain growth, which is thought to enhance themagnetic character of the alloy. Moreover, the electrical resistivity ofthe material is not affected by the thermal treatment. This is animportant parameter, as it is linked to eddy current losses.

Moreover, when ferritic stainless steel is used at elevated temperature,deleterious phases such as chi (χ) sigma (σ) and mu (μ) phases canprecipitate. These phases are in equilibrium in a thermodynamic phasediagram and need time to diffuse. It is therefore important that thematerial does not reside long (a few minutes, 10 minutes at max,preferably 5 minutes) in the annealing temperature range where thesephases are formed.

The rotor lamination stack 31 and/or the stator lamination stack 41 ismanufactured according to a method that will now be described.

In a first step 100, naked sheets of a ferritic material are obtained,e.g. by stamping and cutting material from rollers of raw material inthe case of metallic ferritic material. The geometry of the sheets, inparticular their thickness, depends mainly on the desired type oflamination stack, e.g. either for a rotor or for a stator, and on thesize and power of the electrical machine to be equipped with the stack.

For a magnetic bearing to be used in an Oil & Gas extractor, thethickness of the sheets is typically between 0.05 and 3 mm, andpreferably between 0.1 and 0.3 mm, in order to avoid eddy currents inbearings.

In a second step 200, both sides of each sheet are prepared so as toobtain a determined surface roughness. Indeed, it is essential thatasperities 80 are present on the surfaces of the naked sheets 60. Theseasperities must be in a large enough quantity and their heights andwidths shall be of the order of magnitude of the micron. Typically, theheights and widths of the asperities are of about 2 microns.

Depending on the ferritic raw material used and its manufacturingprocess, there may be the need to prepare the surfaces so as to createthe asperities 80 with dimensions mentioned above. For instance,typically with a ferritic stainless steel material, the surfaceroughness is too smooth and needs to be increased. Traditional sandblasting is unsuitable, as it would make the surface too rough bycreating excessively large asperities.

According to the invention, the surfaces are prepared thanks to achemical attack consisting of an immersion of the naked sheets in acorrosive liquid bath.

Advantageously, the corrosive liquid is a water solution based onhydrofluoric acid. A quick bath of about half an hour at roomtemperature suffices to create asperities of optimum dimensions.

The sheets 60 are then removed from the bath and cleaned so as to removeany particle of corrosive liquid. The asperities 80 are visible on FIG.4 where they have been exaggerated for the sake of clarity.

In a third step 300 of the method of the invention, a chemicallyprotective, electrically insulating material is deposited on at leastone side of each sheet 60.

Advantageously, this material is a fluoropolymer, a material withexcellent ability to resist the chemical aggression of agents such asH₂S, NaCl and CO₂. In particular, this material may be a FluorinatedEthylene Propylene (FEP) or Perfluoroalkoxy Alkane (PFA), rather thanPolytetrafluoroethylene (PTFE).

The electrically insulating and chemically protective material may havethe form of a very thin solid powder with grains of a few microns size(typically 5 to 10 microns), and is deposited on the sheets according toa known method such as pneumatic pulverization or electrostaticdeposition. Alternatively, a colloidal solution can be applied on thesurfaces.

Each sheet 60 is then placed in an oven or under heating lamps at thetemperature exceeding the melting temperature of the insulating materialfor about an hour at maximum, so as to melt the grains. For instance, inthe case of FEP, which has a melting temperature of 260° C., the sheets60 are heated and maintained at a temperature of about 270° C. duringapproximately thirty minutes. This heating temperature is low enough soas not to change the metallurgical structure of the ferritic stainlesssteel AISI 444 and to avoid undesired precipitates as mentioned earlier.

Once melted the insulating material flows into or around the asperities80. Upon cooling, preferably in the ambient air, the insulating materialsolidifies and guarantees a good mechanical anchorage of the insulatingmaterial onto the sheets 60. Hence, coated sheets are obtained, thelayer 70 of insulating material being very thin and uniform all over theface of the sheet. The thickness of the layer obtained according to themethod of the invention is between 5 and 20 microns, depending on thedesired properties of the lamination stack.

Therefore, thanks to the invention, no glue is used to maintain thelayer of insulating material 70 onto the ferritic sheet 60.

In the case where only one face of each sheet 60 is to be covered by alayer 70 of insulating material, it is advantageous to use a coloredinsulating material, so that the side covered is easily recognizableduring the following step according to the invention. In the case wherethe basic insulating material is not colored (this is the case for FEP),a colored organic pigment is be added during the preparation of the rawinsulating material.

In the case where the two faces of each sheet 60 is to be covered by alayer of insulating material, the use of a colored organic pigmentallows to easily control optically the presence of the insulatingmaterial.

It is important to avoid the usage of coloring particles which couldcreate electrical bridges between the sheets and hence the propagationof eddy currents. An organic coloring pigment is therefore appropriate.

In a fourth step 400 of the method of the invention, the coated sheetsare then stacked in a compression tool 50 represented on FIG. 3, betweena lower frame 51 and upper frame 52. It is important that the coatedsheets are placed horizontally so as to be perpendicular to the gravityfield. For sheets with a layer of insulating material 70 on only oneside, it is important that the covered side is the top one.

In a fifth step 500 of the method of the invention, a compressor 53 isthen placed above the upper frame and applies a determined compressionforce F onto the stacked coated sheets. The direction of the compressionforce F is perpendicular to the stacked coated sheets. Thanks to thecompression force, air bubbles which may have been created between thecoated sheets during the stacking operation are expelled. Anotheradvantage of the compression force is to minimize the distance betweenthe sheets 60 of ferritic material while ensuring that there is alwayssome insulating material between two consecutive sheets. Also, thanks tothe compression force, a proper parallelism between the sheets 60 offerritic material is obtained.

The magnitude of the compression force depends on the dimensions and thenumber of stacked coated sheets. Typically, this compression force ischosen in the range from 80 to 200 MPa. The force is built thanks to ahydraulic press. The compression tool 50 further comprises bolts andnuts which are then screwed together until they have taken up completelythe compression force F. The hydraulic press is then removed.

In a sixth step 600 of the method of the invention, while maintainingthe compression force on the stacked coated sheets thanks to the boltsand nuts, the assembly thus obtained is heated up and maintained at atemperature above the melting temperature of the insulating material fora few hours, a time long enough to make sure that each point of theassembly reaches the desired temperature. When FEP is used, duration ofbetween 5 and 8 hours at 350° C. is recommended. The temperature atwhich the compressed stacked coated sheets are maintained shall howeverbe lower than a temperature at which an undesirable intermetallic phaseappears.

When one layer 70 of insulating material belonging to a coated sheet isin direct contact with a naked sheet 60, the insulating material meltsand flows into or on the asperities 80 of the naked sheet 60.

When two layers of insulating material belonging to two consecutivecoated sheets are in contact, the melted insulating material of bothmixes so as to form only one melted layer with melted material stillpresent in or on the asperities of both sheets.

In a seventh step 700 of the method of the invention, the compressedassembly is then cooled down in the ambient air. During the cooling, theelectrical and chemical insulating material solidifies and renders thestack of lamination unitary and rigid thanks to the mechanical anchorageof the insulating material in or on the asperities of the ferriticsheets. No glue or adhesive is used to obtain this result.

The use of solid grains of only a few microns size of chemicallyprotective, electrically insulating material permits to deposit aminimum quantity of it on the naked ferritic sheets, so as to form alayer of a minimum thickness, of about 5 to 20 microns, large enoughjust to isolate electrically two consecutive ferritic sheets and toensure the mechanical stiffness of the lamination stack.

If necessary, further machining operations can be performed on thelamination stack obtained by a method according to the invention, inorder to give the stack its final dimensions.

Consequently, a lamination stack that is formed according to theinvention has an optimal combination of magnetic properties, mechanicalproperties and corrosion resistance.

The invention has been described for its use in magnetic bearings.However, it can be implemented for any other type of rotating electricalmachine such as a motor, an alternator or a generator.

REFERENCE NUMERAL LIST

-   10 assembly-   20 housing-   30 rotor-   31 rotor lamination stack-   32 rotor shaft-   40 stator-   41 stator lamination stack-   42 stator electromagnetic windings-   50 compression tool-   51 lower frame-   52 upper frame-   53 compressor-   60 sheet-   70 layer of chemically protective insulating material-   80 asperities-   F compression force

What is claimed is:
 1. A lamination stack for use in a rotatingelectrical machine, the lamination stack being formed by a methodcomprising: providing a plurality of sheets of ferritic material, eachof the sheets having a first side and a second side and a thickness fromthe first side to the second side, the first and second sides havingasperities each having a height of about two microns; coating at leastone side of each sheet with a chemically protective electricallyinsulating material; stacking the coated sheets; compressing the stackof the coated sheets; heating the compressed stack of coated sheets at atemperature above a melting temperature of the insulating material; andcooling the compressed stack.
 2. The lamination stack according to claim1, wherein the asperities each have a width of about two microns.
 3. Thelamination stack according to claim 1, wherein coating at least one sideof each sheet with a chemically protective electrically insulatingmaterial comprises applying a solid powder to the at least one side ofeach sheet.
 4. The lamination stack according to claim 3, wherein thesolid powder has a grain size of about 5 to 10 microns.
 5. Thelamination stack according to claim 1, wherein the electricallyinsulating material comprises a Fluorinated Ethylene Propylene.
 6. Thelamination stack according to claim 1, wherein the electricallyinsulating material comprises a Perfluoroalkoxy Alkane.
 7. An electricalmachine comprising: a bearing rotor; and a bearing stator, wherein thebearing rotor and/or the bearing stator includes the lamination stackaccording to claim
 1. 8. A lamination stack for use in a rotatingelectrical machine, the lamination stack comprising: a plurality ofsheets of ferritic material, each of the sheets having a first side, asecond side and a sheet thickness from the first side to the secondside, the first and second sides including asperities, each asperityhaving a height of about two microns, and a layer of electricallyinsulating material between adjacent pairs of the ferritic sheets in thestack, wherein the asperities extend into the electrically insulatingmaterial.
 9. The lamination stack according to claim 8, wherein eachasperity has a width of about two microns.
 10. The lamination stackaccording to claim 9, wherein the electrically insulating materialcomprises a Fluorinated Ethylene Propylene.
 11. The lamination stackaccording to claim 9, wherein the electrically insulating materialcomprises a Perfluoroalkoxy Alkane.
 12. The lamination stack accordingto claim 9, wherein the electrically insulating material bonds theadjacent pairs of sheets to each other.
 13. An electrical machinecomprising: a bearing rotor; and a bearing stator, wherein the bearingrotor and/or the bearing stator includes the lamination stack accordingto claim 9.